Application of stent implantation in congenital heart disease

Vascular stenoses and hypoplasia in children are associated with congenital heart disease or occur as residual lesions or complications after surgical repair. These obstructions can occur in arteries or veins of both the pulmonary and systemic circulations. Untreated vascular stenoses cause significant mortality and morbidity and will severely affect surgical outcome. Though conventional surgical repair is effective in relieving some of the vascular stenoses, the operations may be difficult, carries high operative risks or are associated with disappointing outcome. Recurrence of stenoses requiring reintervention is also common. Furthermore, some of the vascular stenoses may be located at surgically inaccessible sites. Immediate results of balloon angroplasty may be satisfactory but sustained relief of obstruction can be achieved in only a certain proportion of patients. Restenosis is common. It is due to elastic recoil of the stenosed vessels, subsequent scarring and fibrosis, or external compression. Stents provide a rigid metallic framework to support the stenosed vessel and prevent restenosis. Following success of stenting in animals, O'Laughlin et al1 reported in 1991 the first success of stent implantation in congenital heart disease. Since then, stents have been used to treat a variety of cardiac lesions including complex congenital heart disease with encouraging results. Types of stents and techniques of implantation Currently a variety of stents can be used in congenital heart disease. They can be categorized according to the material from which they are made, their configuration, size, specific target organ or purpose, presence or absence of a covering membrane, special surface treatment and coatings and drug-eluting properties. But most commonly, and from a pragmatic point of view, classification is based on their delivery mechanism: balloon-expandable vs. self-expanding stents (Figure 1).Figure 1. A:: Balloon-expandable Palmaz stent (A1) before mounting, after mounting onto a balloon dilation catheter (A2) and appearance when partially expanded by the balloon catheter (A3). B: Self-expanding stent held back by a rolling membrane before expansion (B1) and appearance after full expansion on complete withdrawal of the rolling membrane (B2). C is showing unexpanded bare CP stent (C1), expanded bare CP stent (C2) and covered CP stent - unexpanded and expanded appearance (C3).Balloon expandable stents Balloon expandable stents (BES) are mounted on balloons, positioned across the site of obstruction and implanted by inflating the balloon (Figure 2). The size of the inflating balloon determines the diameter of the stent. BES was introduced for treatment of congenital heart disease in 1988 using the prototype stent, the Palmaz stent. The procedure can be performed intraoperatively as a hybrid procedure in young infants, high risk patients, and those requiring concurrent surgical repair.Figure 2. A:: Angiogram on lateral projection showing stenosis in the LPA. B: Fluoroscopy showing positioning of the stent across the stenotic segment of LPA. C: Expansion of the balloon-expandable stent at the stenotic segment of the LPA as indicated by the black arrows. D: Angiogram on lateral projection showing the full expansion of the stenotic segment of LPA after stenting. Arrows indicate position of the guidewire.The Plamaz stent has a closed cell design and consists of a stainless steel tube with a slotted tube cut by a laser to form 7 cells per row. They have been the most commonly used stents in paediatric cardiology because of high radial force, optimal scaffolding with a regular surface, accurate placement and oversizing properties to accommodate growth. However, the shortcomings include marked foreshortening, rigidity, flaring of sharp edges which causes vessel injury and balloon rupture and the unsatisfactory flexibility due to its closed cell design. To overcome the lack of flexibility, the Corinthian series (Johnson and Johnson Int. Systems, Warren, NJ, USA) and the Genesis™ stent (sigmoidal hinge connection, Johnson & Johnson) provide improved flexibility by adding a flexconnector which is either a U-,V-, S- or N-shaped element that plastically deform during bending. The Genesis stent is available in unmounted or pre-mounted state. The more modern stents are stents with open cell design which has a geometry that does not connect consistently throughout the stent. With expansion, the individual cells merge to form larger open areas, thus, there is no consistent shape of cells. Examples of the open cell design are the GuidantHerculink™ and Omnilink™ (Guidant, Santa Clara, CA, USA), Intrastent™ DoubleStrut™, Mega™ LD and Maxi™ LD series (EV3 Inc., Plymouth, MN, USA).2 These “modern stents” possess characteristics that are advantageous for their use in congenital heart disease: (1) less foreshortening, (2) conformable, (3) less likely to cause balloon rupture, (4) allow access to jailed branches, (5) more flexible compared with stents with close cell design, and (6) their rounded edges minimize trauma to vessel wall. A few shortcomings include reduced radial strength beyond 15-mm diameter of Doublestrut™ and a higher incidence for protrusion of intimal tissue or plaque between struts which might impact long-term stent performance. In contrast to stents with the slotted tube design, the Cheatham platinum (CP) stent (Figure 1) used a welded tube manufactured from a wire which is bent and welded to a cylindrical meshwork forming the stent. This results in more adjustable flexibility and a wide range of size and length, but the radial strength is less than the ones with slotted tube. The CP stent in made from 90% platinum and 10% iridium with the addition of gold soldering at each welded spot to increase the total strength of the weld. The number of zigs affects the final diameter, the degree of shortening and the profile of the stent. It is available in 6 and 8 rows, the 8-zig configuration is used most as this shortens much less than the 6-zig at equivalent diameters and can be dilated up to 28 mm (instead of 18 mm for 6-zig) with foreshortening of 22%-28%.3 The stent has excellent visibility on fluoroscopy and good radial strength even at large diameters. For introduction, sheaths 2F larger than that required for the balloon are required as they have slightly larger profile than other stents. The CP stent is magnetic resonance (MR) compatible and its round edges causes less trauma to vessel wall and balloon. The covered stents The CP stents are available in a covered version with an outer polytetrafluoroethylene (PTFE) membrane (Figure 1). This covering is initially 7 mm in diameter and will stretch over the range diameters of expansion, usually from 12 mm-24 mm and will always be taut over the stent when expanded. To date, the covered CP stent is the most widely used covered stent in patients with congenital heart disease.3,4 Self-expanding stent Materials for the self-expanding stents should exhibit large elastic strains. The most widely used material is nitinol which is a nickel-titanium alloy that can be recovered from elastic deformation of up to 10%. The more conventional materials such as stainless steel (Cook “Z stent”) or cobalt-based alloy (BSC“Wallstent”) have limited elastic range compared with nitinol and are therefore limited in use. Commonly used self-expanding stents in paediatric cardiology include the Wallstent, the SMART stent, Strecker stent, Dynamlink, Portégé GPS stent and Cook Zilver stent (Figure 1B). Self-expanding stents are constrained a covering membrane and can be mounted on and by a delivery They are more flexible than BES and can and lesions but their radial strength is than that of withdrawal of the membrane will the stent which will to its diameter and shape to the stenotic of the of they may a expansion the first of The of self-expanding stents is the of more intimal due to the of of nitinol which cell in in is that they be dilated to accommodate their use in children are they are implanted in that not or are a surgical a or a implantation for specific lesions or pulmonary arteries or pulmonary stenoses may be congenital or and may occur as a stenosis or hypoplasia in a wide variety of congenital heart disease. They may also occur in and congenital stenosis occur after surgical repair of of complex pulmonary repair of and after or the In of the after or may cause pulmonary of surgical repair of pulmonary stenoses are disappointing and balloon dilation relief in only of The recoil of tissue and subsequent scarring in restenosis. The to in to a to complications or even vascular rupture pulmonary stent implantation can overcome the of and balloon its in and pulmonary stenoses has both balloon-expandable and self-expanding stents have been the is more and in children because they can be dilated to accommodate growth. than the most widely used and stents (Johnson & Johnson Systems, Warren, USA), the Genesis the and the CP stent are also BES that can be results of stent implantation in and pulmonary stenoses have been They are to be and effective in the vessel diameter, the across the vessels, and complications stent and stent that significant of pulmonary stent in only of but they are to A higher of up to may occur in those with reported long-term complications stent and intimal of branches, stent vessel rupture, of stent, pulmonary and In the the mortality and morbidity the and of stent as the treatment of for pulmonary of The mortality for surgical repair for of is only in patients of but slightly higher in and However, significant at a of 10% in most surgical after surgical to be The but may occur after surgical repair. of balloon in relieving of made a treatment and is to treat It is that relief of obstruction by balloon in both and are and can be achieved in of It is in patients with because of the of tissue which is in of balloon are much less in and to of tissue to a much higher incidence of vs. and reintervention compared with surgical repair. is as treatment of for and young The complications after balloon are and in of and there is a risk of rupture This high incidence as compared with is because balloon dilation the and of the may also occur in These the wall and the patients to In of the segment that is in to the diameter is also to or even from Though of and in the a few may and or implantation of a covered stent. after balloon is usually due to elastic or It in of patients, will likely the and of balloon stent implantation in or was introduced in the and success has to its on an large over the (Figure The of using a stent is that elastic the and allow the full dilation to be in These the risk of and The risk of from a series of is and the can be by The has showing a segment of with some in the the stenosis as Fluoroscopy showing and full expansion of the balloon-expandable stent positioned at the D: Angiogram showing full expansion of the segment after implantation of Plamaz large stents are for The first and most commonly used stent was the Palmaz stents in the and However, a number of stents have been introduced for this The and LD are more flexible and can be dilated up to diameter of The CP stent, the covered CP have been used with encouraging results. It has the first in some The Genesis is also a stent for stents which are of profile are also for more is It is that the covered CP stents are for use in associated with or in complex there is a high risk of are lesions with and the presence of a concurrent The covered stent can also as a or a when vascular rupture or complications The with the covered stents is the covering of vessels, the arteries which may to However, the usually the of the from the the in stenting of is to occur but in that in the of stent of the covered stent at a segment might of the arteries has been not to cause as to be of can also a (Figure or a and With the reported success complications of stent have been reported in up to of patients in a on over complications include vessel stent and is the most with an incidence up to Balloon rupture with stent may be by of the by the use of stents with and CP stents and the use of balloon to access the and from the from use of large sheaths can be in by using vascular the or Vascular CA, use of delivery of or even higher can be used to stents with larger of or tissue can also risk of have been reported to be to pulmonary The placement of between the pulmonary and pulmonary arteries are to some complex cardiac However, surgical to the are always required because of the over and on an at and of and with obstruction the most for surgical to include external by the and of a intimal of children also These can occur in both and is a the and subsequent have a than the because and at more to an and characteristics at of balloon dilation of obstruction are or with of the of the implantation the has been to be a to stenosis and surgical With effective of across the stenosed and increase in the diameter of the stenotic the of the can be et that with stent implantation in from surgical reintervention was at at and at and of children was include stent and of The long-term of pulmonary on the after implantation a In contrast to a stent in the the introduced pulmonary implantation MN, is effective in both the stenosis and of the with excellent and long-term In the which has been used in implantation in has also results when implanted in the pulmonary has the stent as the of for in patients. stenting as a treatment In with of such as those in of with complex pulmonary with arteries repair may either be not or a has to be include a pulmonary a a or a or stenting. is a stent to pulmonary and of pulmonary the can also be after of pulmonary or in of the size of pre-mounted stents or the Palmaz Genesis stents to 12 are These stents are at the of subsequent include stent which can occur at due to the high of the at the the of stent to scaffolding and subsequent of the implantation may be in such may also is after stenting. and systemic obstruction obstruction the or may be congenital or after surgical for catheter or and after heart at the sites. obstruction is also significant in patients the or of obstruction in the veins is not only for relief but also to vascular access for relief of systemic obstruction and procedure obstruction is difficult, and may be balloon dilation only relief of implantation an treatment balloon-expandable and self-expanding stents have been The procedure is and effective in relief of obstruction and systemic of the be on long-term and of the required a degree of intimal in some patients. significant complications on long-term Balloon-expandable stents that can be dilated to diameters should be the treatment of for systemic The more modern stents the CP or LD series which are less rigid and less are obstruction obstruction in congenital pulmonary total pulmonary or after their surgical repair. 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